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Fan Affinity Law — Airflow vs Speed

Also known as fan laws · VFD airflow change

Q2Q1=N2N1\frac{Q_2}{Q_1} = \frac{N_2}{N_1}

Enter your known values, leave one input blank, and solves for the missing one. Try different units for next level excitement!

Learning zone

Air is a fluid like any other, so a fan wheel obeys the same first affinity law as a pump impeller: 20% more rpm buys 20% more cfm. A balancer measuring 8000 cfm at 900 rpm who needs design airflow speeds the wheel to 1080 rpm and reads 9600 cfm. On belt-driven units the adjustment is made not with a dial but with sheaves — swapping the motor pulley for one 20% larger raises fan rpm by 20%, which is why an air-balance report always records both sheave sizes.

The universal field mistake is treating this as free. Airflow is linear but brake horsepower is cubic, so the 20% re-sheave that fixes the airflow deficiency raises fan power by 73% and will happily trip an already fully loaded motor. Check the motor nameplate amps before you cut the belt, not after.

Fan Affinity Law — Airflow vs Speed
Q2Q1=N2N1\frac{Q_2}{Q_1} = \frac{N_2}{N_1}
Where
  • Q1Q_1= Airflow at speed 1
  • N1N_1= Fan speed 1
  • Q2Q_2= Airflow at speed 2
  • N2N_2= Fan speed 2
Missing one of these? Work it out first, then come back
Part of this formula set

The pump and fan affinity laws

7 formulasHow flow, head and power scale with impeller speed and diameter — the first, second and third power relationships behind every VFD retrofit.